Switching power supply circuit and overcurrent protection method for the switching power supply circuit

ABSTRACT

A switching power supply circuit comprises a switching element for controlling the on/off state of an input power supply according to a drive pulse generated on the basis of a clock pulse having a prescribed period. Breakdown of circuit elements due to current superposition is effectively prevented by detecting current flowing through the switching element and flywheel current, and comparing these detected results with a first and second reference value, respectively. An overcurrent detection circuit detects that the current flowing through a transistor for controlling the on/off state of the input power source has reached a first reference value, a flywheel current detection circuit detects flywheel current, an overcurrent protection operation is performed for turning off the transistor based on the detected output of the overcurrent detection circuit, and the transistor is turned on during the overcurrent protection operation at the timing of the clock pulse after the flywheel current detected by the flywheel current detection circuit has reached the second reference value.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a switching power supply circuitand an overcurrent protection method for the switching power supplycircuit, and particularly relates to a switching power supply circuitand an overcurrent protection method for the switching power supplycircuit whereby reliable protection is provided against breakdown ofcircuit elements and the like due to overcurrent.

[0003] 2. Description of the Related Art

[0004]FIG. 9 is a circuit diagram depicting an example of a conventionalswitching power supply circuit. The switching circuit depicted in FIG. 9constitutes a step-down DC-DC converter. In this switching circuit, afield-effect transistor (hereinafter referred to simply as “transistor”)20 is used as a switching element. A direct current outputted from aninput power supply 10 is stepped down by controlling the on/off state ofthe transistor 20 by means of a drive pulse DP′ from a control circuit100, and the stepped-down current is rectified by a rectifier circuitmade up of an inductor 30 and diode 40. This rectified current isoutputted from an output terminal 50.

[0005] In this conventional switching power supply circuit, a capacitor60 smoothes the direct current voltage outputted from the outputterminal 50, resistors 70 and 80 divide and detect the direct currentvoltage outputted from the output terminal 50, and the voltage detectedby the resistors 70 and 80 is applied to the control circuit 100 as afeedback signal FB.

[0006] The control circuit 100 comprises a PWM (Pulse Width Modulation)circuit 110 and an overcurrent protection circuit 120. The PWM circuit110 generates a drive pulse DP (PWM pulse) in synchronism with a clockpulse CK at a prescribed period for controlling the on/off state of thetransistor 20 based on a feedback signal FB.

[0007] The overcurrent protection circuit 120 continually monitors thecurrent I-H flowing through the transistor 20. When the current I-Hflowing through the transistor 20 exceeds a pre-set prescribed value,the overcurrent protection circuit 120 operates so as to mask the drivepulse DP outputted from the PWM circuit 110 and turn the transistor 20off.

[0008] In FIG. 9, I-L indicates a flywheel current that flows throughdiode 40 and inductor 30 when the transistor 20 is turned off.

[0009]FIG. 10 is a circuit diagram depicting an example of theovercurrent protection circuit 120 of the switching power supply circuitdepicted in FIG. 9; FIG. 11 is a circuit diagram depicting an example ofthe overcurrent detection circuit 121 depicted in FIG. 10; and FIG. 12is a timing chart depicting the operation of the circuits depicted inFIGS. 9 through 11.

[0010] In FIG. 11, the current I-H flowing through the transistor 20 isdetected by the circuit that includes the resistor R1 and transistorFET1.

[0011] In FIG. 11, the transistor 20 and transistor FET1 constitute acurrent mirror circuit. From the terminal T2, the drive pulse DP′ forcontrolling the on/off state of the transistor 20 is applied to thegates of both the transistor 20 and the transistor FET1, and a current(I-H)/n, which is the 1/nth of the current I-H flowing through thetransistor 20, flows into the resistor R1.

[0012] Consequently, a detected voltage VId that corresponds to thecurrent I-H flowing through the transistor 20 can be obtained from thejunction of the resistor R1 with the transistor FET1.

[0013] This detected voltage VId is applied to a comparator CO, and withthe aid of the comparator CO, the detected voltage VId is compared withthe voltage (Vin−V1) obtained by subtracting from the voltage Vin of theinput power supply 10 a prescribed reference voltage V1 for detectingovercurrent (see FIG. 12A).

[0014] When the detected voltage VId becomes smaller than the voltage(Vin−V1), specifically, when the current I-H flowing through thetransistor 20 increases above a prescribed value, an overcurrentdetection signal OCD is outputted from the comparator CO (see FIG. 12B).

[0015] This overcurrent detection signal OCD is usually low-level, asdepicted in FIG. 12B. However, it becomes high-level when the currentI-H flowing through the transistor 20 exceeds a prescribed value.

[0016] The overcurrent detection signal OCD outputted from theovercurrent detection circuit 121 is outputted from the terminal T1depicted in FIG. 11 and applied to the set terminal S of the flip-flop123 depicted in FIG. 10.

[0017] The clock signal CK from the terminal 122 used by the PWM circuit110 is applied to the reset terminal R of the flip-flop 123, which goesto “high-level” when the overcurrent detection signal OCD from theinverted output terminal thereof is “low-level,” and generates alow-level gate signal GS when the overcurrent detection signal OCD goesto “high-level” (see FIG. 12E). This gate signal GS is applied to an ANDcircuit 125.

[0018] A drive pulse DP (see FIG. 12D) outputted from the PWM circuit110 is applied from the terminal 124 to the other input terminal in theAND circuit 125. As a result, when the overcurrent detection signal OCDis brought to a “high-level”, the AND circuit 125 masks the drive pulseDP outputted from the PWM circuit 110. In other words, the AND circuit125 prohibits outputting of the drive pulse DP from the PWM circuit 110(see FIG. 12F).

[0019] The output of the AND circuit 125 is inverted by an inverter 126and is outputted from a terminal 127 as a drive pulse DP′ forcontrolling the on/off state of the transistor 20 (see FIG. 12G).

[0020] Also, the time period Tocp depicted in FIG. 12E consists of theoperating period of the overcurrent protection circuit 120.

[0021] Also, in the above-mentioned circuit example, because a P channelMOS is used as the transistor 20, the transistor 20 is turned on whenthe drive pulse DP′ is “low-level.”

[0022] The above-mentioned overcurrent protection circuit 120 also hassuch problems as are depicted in FIG. 13.

[0023] Specifically, the minimum duty in the drive pulse DP′ forcontrolling the on/off state of the transistor 20 is determined from theperformance limit of the circuit. Therefore, as depicted in FIG. 13E,the minimum value of the on-duty time wherein the drive pulse DP′ can bemaintained at low-level is restricted to Tmin.

[0024] Because of this, as depicted in FIG. 13F, for example, even ifthe output voltage goes to zero due to load shorting and the like andthe current I-H flowing through the transistor 20 exceeds the pre-setprescribed value Vocp (see FIG. 13A), the transistor 20 does notimmediately turn off during the minimum duty time Tmin. The current I-Halso rises abruptly when the current I-H exceeds the saturation currentIsa of the inductor 30.

[0025] At the time when the next clock signal CK rises while theovercurrent protection circuit 120 is in operation, if the flywheelcurrent I-L is not sufficiently attenuated as depicted by the dash linein FIG. 13A, current superposition occurs in the inductor 30 (see FIG.13A). As a result of these facts, the circuit can break down in a worstcase.

[0026] These facts are likely to occur in such an occasion where thefrequency of the clock signal CK is set high, i.e., at a few MHz and theload is extremely large.

[0027] Techniques for overcoming the problems caused by theabove-mentioned current superposition have been disclosed in JapanesePatent Application Publication Nos. 07-46828 and 11-341791, and in U.S.Pat. No. 5,808,455.

[0028] To prevent operating lag in the overcurrent protection circuit incases in which the switching frequency is high, the switching powersupply circuit disclosed in Japanese Patent Application Publication No.07-46828 is configured so as to minimize the effects of lag time betweenthe time an overcurrent state is detected and the time the transistorturns off, by lowering the switching frequency when the output voltagefalls to a prescribed level.

[0029] The switching power supply circuit disclosed in Japanese PatentApplication Publication No. 11-341791 is configured to perform controlso as to lower the detection level of the overcurrent when the outputvoltage further declines due to a short-circuit or the like, in additionto having the configuration of the switching power supply circuitdisclosed in Japanese Patent Application Publication No. 07-46828.

[0030] The switching power supply circuit disclosed in U.S. Pat. No.5,808,455 focuses on the problems of the conventional configurationdepicted in FIG. 1 of the same reference, specifically, the problem ofpower dissipation in a conventional configuration in which a currentdetecting resistor R is connected in series with an inductor L, andprovides a configuration whereby overcurrent can be detected whileminimizing power dissipation by disposing a current detection resistor Ron the low side as depicted in FIG. 2 of the same reference. As depictedin FIG. 3 of the same reference, this technique is furthermore designedto effectively restrict overcurrent and minimize breakdown in circuitelements by extending the electrical discharge cycle until the inductorcurrent IL reaches I LIMIT 2 after the inductor current IL exceeds aprescribed reference value I LIMIT 1. Also, these two reference valuesare set and compared using a Schmitt trigger U1.

[0031] A configuration whereby a pre-set timer circuit is started upondetection of overcurrent by the overcurrent detection circuit and thedrive pulse for controlling the switching on and off of the switchingpower supply circuit is masked during the timer circuit period has alsobeen considered as another technique for overcoming the problems of theabove-mentioned current superposition.

[0032] The above-mentioned current superposition can also be preventedand circuit damage due to overcurrent can be prevented in thisconfiguration if the timer circuit period is set sufficiently long.

[0033] However, both of the switching power supply circuits disclosed inthe above-mentioned Japanese Patent Application Laid-open Nos. H7-46828and H11-341791 have the problem of reduced responsiveness, because theyreduce the switching frequency when overcurrent is detected, and aconfiguration that uses a timer circuit has problems whereby the timingperiod of the timer circuit is difficult to set, and the switching cycleof the switching power supply circuit is effectively lengthened,yielding inadequate performance if the set timing period is too long.

[0034] Also, in a configuration that uses a timer circuit, the user doesnot usually know the saturation current value of the inductor of theswitching power supply circuit being used. Therefore, theabove-mentioned timing period of the timer circuit must actually be setquite long to allow a margin, in which case problems such as follows maybe encountered.

[0035] 1) Startup time is lengthened, and

[0036] 2) Response to sudden load fluctuations is delayed

[0037] Also, because overcurrent detection is delayed by having theovercurrent detection resistor R disposed on the low side, and becauseboth a positive power supply and negative power supply are needed inorder to utilize a Schmitt trigger U1 provided with two reference valuesin a configuration in which the overcurrent detection resistor R isdisposed on the low side, the switching power supply circuit disclosedin U.S. Pat. No. 5,808,455 has many aspects that demand improvement inorder to obtain a simple circuit structure.

SUMMARY OF THE INVENTION

[0038] Therefore, an object of the present invention is to provide aswitching power supply circuit and overcurrent protection method for theswitching power supply circuit that has rapid startup time and excellentresponse to sudden load fluctuations, and that is capable of reliablyprotecting against a breakdown due to overcurrent in circuit elementsand the like with a simple structure.

[0039] To achieve the above objects, the present invention provides aswitching power supply circuit having a switching element forcontrolling on/off state of an input power supply according to a drivepulse generated on the basis of a clock pulse having a prescribedperiod, wherein an electric current is allowed to flow from the inputpower supply to a load by controlling the switching element to be in onstate, and a flywheel current is allowed to flow to the load bycontrolling the switching element to be in off state, in which theswitching power supply circuit comprises overcurrent detecting means fordetecting that the current flowing through the switching element hasreached a first reference value; flywheel current detecting means fordetecting that the flywheel current has reached a second referencevalue; and an overcurrent protection circuit for performing anovercurrent protection operation of turning the switching element offbased on a detected output of the overcurrent detecting means, and forturning the switching element on during the overcurrent protectionoperation at a timing of the clock pulse after the flywheel currentdetected by the flywheel current detecting means has reached the secondreference value.

[0040] With the above configuration, the flywheel current detectingmeans may detect the flywheel current based on an output of a currentdetection resistor provided in a flywheel current path.

[0041] The flywheel current detecting means may comprise a secondswitching element provided in the flywheel current path and being turnedon and off in synchronism with the first-mentioned switching element,and detects the flywheel current using a resistance of the secondswitching element in on state.

[0042] The flywheel current detecting means may further comprise a thirdswitching element provided in the flywheel current path and constitutesa current mirror together with the second switching element that isturned on and off in synchronism with the first-mentioned switchingelement, whereby the flywheel current detecting mean detects theflywheel current based on the current flowing through the thirdswitching element.

[0043] The second reference value may be set to a value of zero orabove.

[0044] Further, a switching power supply circuit according to anotheraspect of the present invention comprises a switching element providedwith a source terminal, a drain terminal and a gate terminal; a powersupply source connected to the source terminal; a load connected to thedrain terminal; a control circuit connected to the gate terminal; aninductor disposed in a current path from the drain terminal to the load;a first current detection resistor disposed in a current path from thesource terminal to the load; a first comparator connected to the firstcurrent detection resistor; a second current detection resistor disposedin a current path from the drain terminal to the ground terminal; and asecond comparator connected to the second current detection resistor.

[0045] With this configuration, the second comparator may use a groundpotential or positive potential as a reference.

[0046] Further, still another aspect of the present invention providesan overcurrent protection method for a switching power supply circuithaving a switching element for controlling on/off state of an inputpower supply according to a drive pulse generated on the basis of aclock pulse having a prescribed period, wherein an electric current isallowed to flow from the input power supply to a load by controlling theswitching element to be in on state, and a flywheel current is allowedto flow to the load by controlling the switching element to be in offstate, in which the overcurrent protection method comprises detectingthat the current flowing through the switching element has reached afirst reference value; detecting with an overcurrent detecting meansthat the current flowing through the switching element has reached afirst reference value; detecting with flywheel current detecting meansthat the flywheel current has reached a second reference value;performing an overcurrent protection operation for turning off theswitching element based on a detected output of the overcurrentdetecting means; and turning on the switching element during theovercurrent protection operation at a timing of the clock pulse afterthe flywheel current detected by the flywheel current detecting meanshas reached the second reference value.

[0047] In accordance with the present invention, a switching powersupply circuit that has rapid startup time and excellent response tosudden load fluctuations, and that is capable of reliably protectingagainst breakdown due to overcurrent in circuit elements and the likewith a simple structure can be provided, as can an overcurrentprotection method for the switching power supply circuit, by adopting aconfiguration in which a switching power supply circuit has a switchingelement for controlling the on/off state of an input power supplyaccording to a drive pulse generated on the basis of a clock pulsehaving a prescribed period, for allowing electric current to flow fromthe input power supply to a load by the on-control to the switchingelement, and allowing flywheel current to flow to the load by theoff-control to the switching element; wherein the switching power supplycircuit is equipped with overcurrent detecting means for detecting thatthe current flowing through the switching element has reached a firstreference value; flywheel current detecting means for detecting that theflywheel current has reached a second reference value; and anovercurrent protection circuit for performing an overcurrent protectionoperation whereby the switching element is turned off based on thedetected output of the overcurrent detecting means, and for turning theswitching element on during the overcurrent protection operation at thetiming of the clock pulse after the flywheel current detected by theflywheel current detecting means has reached the second reference value.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In the accompanying drawings:

[0049]FIG. 1 is a circuit diagram schematically depicting the structureof a switching power supply circuit according to the switching powersupply circuit and overcurrent protection method for the switching powersupply circuit according to the present invention;

[0050]FIG. 2 is a circuit diagram depicting the detailed structure ofthe overcurrent protection circuit and current detection circuit of thecontrol circuit depicted in FIG. 1;

[0051]FIGS. 3A through 3j are timing charts describing the overcurrentprotection operation of the switching power supply circuit depicted inFIGS. 1 and 2;

[0052]FIG. 4 is a circuit diagram depicting another embodiment of theswitching power supply circuit according to the present invention;

[0053]FIG. 5 is a circuit diagram depicting the detailed structure ofthe overcurrent protection circuit and current detection circuit of thecontrol circuit depicted in FIG. 4;

[0054]FIG. 6 is a circuit diagram depicting a modification of thecurrent detection circuit depicted in FIG. 5;

[0055]FIG. 7 is a circuit diagram depicting a modification of thecurrent detection circuit 230 depicted in FIG. 2;

[0056]FIG. 8 is a circuit diagram depicting another modification of thecurrent detection circuit 230 depicted in FIG. 2;

[0057]FIG. 9 is a circuit diagram depicting an example of a conventionalswitching power supply circuit;

[0058]FIG. 10 is a circuit diagram depicting an example of theovercurrent protection circuit of the switching power supply circuitdepicted in FIG. 9;

[0059]FIG. 11 is a circuit diagram depicting an example of theovercurrent protection circuit depicted in FIG. 10;

[0060]FIGS. 12A through 12G are timing charts depicting the operation ofthe circuits depicted in FIGS. 9 through 11; and

[0061]FIGS. 13A through 13F are timing charts describing the problems ofthe conventional overcurrent protection circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0062] A detailed description of the switching power supply circuit andthe overcurrent protection method for the switching power supply circuitaccording to the present invention will be given with reference to theaccompanying drawings.

[0063]FIG. 1 is a circuit diagram schematically depicting the structureof a switching power supply circuit according to the switching powersupply circuit and overcurrent protection method for the switching powersupply circuit according to the present invention

[0064] In FIG. 1, the same symbols are used as are used in FIG. 9 forconvenience in describing components that fulfill the same functions asin the conventional circuit depicted in FIG. 9.

[0065] In FIG. 1, the switching power supply circuit constitutes astep-down DC-DC converter the same as the conventional switching powersupply circuit depicted in FIG. 9. In this switching power supplycircuit, a transistor (field-effect transistor) 20 acts as a switchingelement, and an input power supply 10 such as a battery acts as a powersupply source for the DC-DC converter. A direct current outputted fromthe input power supply 10 is stepped down by controlling the on/offstate of the transistor 20 according to a drive pulse DP′ from a controlcircuit 200, the stepped-down direct current is then rectified by arectifier circuit made up of an inductor 30 and a diode 40, and therectified current is outputted from an output terminal 50 to a load (notshown).

[0066] The transistor 20 is provided with a source terminal, a drainterminal and a gate terminal. The input power supply 10 is connected tothe source terminal, a load (not shown) is connected to the drainterminal via the inductor 30, and the control circuit 200 is connectedto the gate terminal.

[0067] The inductor 30 is an element that stores an electric currentsupplied from the input power supply 10 as magnetic field energy whenthe transistor 20 is turned on, and discharges the stored magnetic fieldenergy as an electric current when the transistor 20 is turned off. Theinductor 30 is disposed in the current path between the drain terminalof the transistor 20 and the load.

[0068] The capacitor 60 is designed for smoothing the direct currentvoltage outputted from the output terminal 50, and the resistors 70 and80 are designed for dividing and detecting the direct current voltageoutputted from the output terminal 50. The voltage detected by theresistors 70 and 80 is applied to the control circuit 200 as a feedbacksignal FB.

[0069] The resistor 90 is a current detection resistor for detecting aflywheel current I-L flowing via the diode 40 and inductor 30 when thetransistor 20 is turned off, and is disposed in the current path betweenthe drain terminal of the transistor 20 and the ground terminal mountedon the converter.

[0070] The control circuit 200 comprises a PWM (Pulse Width Modulation)circuit 210, an overcurrent protection circuit 220, and a currentdetection circuit 230.

[0071] The PWM circuit 210 generates a drive pulse DP (PWM pulse) insynchronism with a clock pulse CK with a prescribed period. The drivepulse DP controls the on/off state of the transistor 20 based on afeedback signal FB.

[0072] The current detection circuit 230 uses a voltage drop across theresistor 90 to detect the flywheel current I-L flowing via the diode 40and inductor 30.

[0073] The overcurrent protection circuit 220 constantly monitors thecurrent I-H flowing through the transistor 20, performs an overcurrentprotection operation for masking the drive pulse DP outputted from thePWM circuit 210 when the current I-H flowing through the transistor 20exceeds a pre-set prescribed value, and performs control during theovercurrent protection operation, for canceling the ON state of thetransistor 20 until the flywheel current I-L detected by the currentdetection circuit 230 is brought to substantially zero.

[0074]FIG. 2 is a circuit diagram depicting the detailed structure ofthe overcurrent protection circuit 220 and current detection circuit 230of the control circuit 200 depicted in FIG. 1.

[0075] In FIG. 2, the overcurrent protection circuit 220 comprises anovercurrent detection circuit 221 for constantly monitoring the currentI-H flowing through the transistor 20 and outputting an overcurrentdetection signal OCD when the current I-H flowing through the transistor20 reaches a pre-set first reference value; a flip-flop 213 in which theovercurrent detection signal OCD outputted from the overcurrentdetection circuit 221 is applied to a set terminal S thereof and theclock signal CK from the terminal 212 is applied to a reset terminal Rthereof, an AND circuit 214 to which the drive pulse DP (PWM pulse)outputted from the PWM circuit 210 and the inverted output GS1 of theflip-flop 213 are applied; an inverter 216 for reversing the flywheelcurrent detection signal IF outputted from the current detection circuit230; an edge detection circuit 217 for detecting the rising edge of thesignal outputted from the inverter 216; a flip-flop 218 in which theovercurrent detection signal OCD outputted from the overcurrentdetection circuit 221 is applied to the set terminal S thereof, and theoutput of the edge detection circuit 217 is applied to the resetterminal R thereof, an AND circuit 215 to which the output of the ANDcircuit 214 and the inverted output GS2 of the flip-flop 218 areapplied; and an inverter 219 for inverting the output of the AND circuit215 and applying it to the gate of the transistor 20 as a drive pulseDP′.

[0076] The overcurrent detection circuit 221 may preferably beconfigured as depicted in FIG. 11 so that a current mirror isconstituted with the transistor 20 and transistor FET1, and a currentdetection resistor is mounted in the branch in which the transistor FET1is disposed so as to realize a minimal power dissipation. However, aconfiguration for detecting the current I-H may also be adopted whereina resistor for current detection is disposed in the current path fromthe source terminal of the transistor 20 to the load.

[0077] The current detection circuit 230 comprises an operationalamplifier 231 in which the junction of the diode 40 with the resistor 90is connected to the positive terminal, and the power supply 232 forgenerating a reference voltage V2 (second reference value) for detectingthe flywheel current I-L is connected to the negative terminal.

[0078] An arrangement may be such that the PWM circuit 210 has the samestructure as the PWM circuit 110 depicted in FIG. 9, and the overcurrentdetection circuit 221 has the same structure as the overcurrentdetection circuit 121 depicted in FIG. 11.

[0079] In the circuit depicted in FIG. 2, the current detection circuit230 detects the flywheel current I-L flowing via the diode 40 andinductor 30 by means of the operational amplifier 231 when thetransistor 20 is turned off.

[0080] Specifically, an electric potential that corresponds to a voltagedrop across the resistor 90 by the flywheel current I-L is inputted tothe positive terminal of the operational amplifier 231 and an electricpotential (GND−V2), in which a reference voltage V2 is subtracted fromthe ground potential, is inputted to the negative terminal thereof.Therefore, a high-level flywheel current detection signal IF isoutputted from the operational amplifier 231 when the voltage dropacross the resistor 90 by the flywheel current I-L becomes greater thanthe reference voltage V2.

[0081] If the reference voltage V2 is made to be zero, a high-levelflywheel current detection signal IF is outputted from the time theflywheel current I-L is generated until the time it decreases to zero.If the reference voltage V2 is a constant value, a high-level flywheelcurrent detection signal IF is outputted from the time when the flywheelcurrent I-L is equal to or above a current that corresponds to theconstant value until the time it decreases to a current corresponding tothe constant value.

[0082] The flywheel current detection signal IF is inverted by theinverter 216 of the overcurrent protection circuit 220, the rising edgethereof is detected by the edge detection circuit 217, and an edgedetection pulse IFE is outputted from the edge detection circuit 217 atthe detected timing. The edge detection pulse IFE is applied to thereset terminal R of the flip-flop 218.

[0083] Specifically, the overcurrent detection signal OCD outputted fromthe overcurrent detection circuit 221 is applied to the set terminal Sof the flip-flop 218, and the edge detection pulse IFE is applied to thereset terminal R thereof, so the flip-flop 218 is set at the timing atwhich overcurrent in the current I-H flowing through the transistor 20is detected by the overcurrent detection circuit 221, and is reset atthe timing at which the flywheel current detection signal IF isdecreased to a value corresponding to the reference voltage V2.

[0084] Also, the overcurrent detection signal OCD outputted from theovercurrent detection-circuit 221 is applied to the set terminal S ofthe flip-flop 213, and the clock pulse CK is applied to the resetterminal R thereof, so that the flip-flop 213 is set at the timing atwhich overcurrent in the current I-H flowing through the transistor 20is detected by the overcurrent detection circuit 221, and is reset atthe clock pulse CK, specifically, at the timing at which the next drivepulse DP rises from the PWM circuit 210.

[0085] The drive pulse DP outputted from the PWM circuit 210 is thenblocked while the flip-flop 213 is set by the AND circuit 214, and isfurthermore blocked while the flip-flop 218 is set by the AND circuit215. As a result, outputting of the drive pulse DP outputted from thePWM circuit 210 is blocked from the timing at which overcurrent isdetected by the overcurrent detection circuit 221 until the timing atwhich the flywheel current detection signal IF falls to a valuecorresponding to the reference voltage V2.

[0086] Consequently, the transistor 20 is not turned on from the timingat which overcurrent is detected by the overcurrent detection circuit221 until the timing at which the next clock pulse CK rises after thetiming at which the flywheel current detection signal IF falls to avalue corresponding to the reference voltage V2.

[0087]FIG. 3 is a timing chart describing the overcurrent protectionoperation of the switching power supply circuit depicted in FIGS. 1 and2.

[0088] In the switching power supply circuit depicted in FIGS. 1 and 2,if shorting occurs in the load and the output voltage Vout goes to zeroas depicted in FIG. 3B, the current I-H flowing through the transistor20 gradually increases from the timing at which the drive pulse DPoutputted from the PWM circuit 210 rises as depicted in FIG. 3F, andexceeds the overcurrent detection value Iocp of the overcurrentdetection circuit 221 as depicted in FIG. 3A.

[0089] A high-level overcurrent detection signal OCD is outputted fromthe overcurrent detection circuit 221 while the current I-H flowingthrough the transistor 20 exceeds over the overcurrent detection valueIocp (FIG. 3C). Since the overcurrent detection signal OCD is outputtedat the time when the current I-H reaches the overcurrent detection valueIocp, early detection of overcurrent is effectively performed.

[0090] The flywheel current I-L then flows via the diode 40 when thetransistor 20 is turned off, as indicated by the dash line in FIG. 3A.

[0091] The flywheel current I-L is detected by the overcurrent detectioncircuit 230, and as depicted in FIG. 3G, a high-level flywheel currentdetection signal IF is generated from the current detection circuit 230from the time the flywheel current is generated until the time itdecreases to zero. Incidentally, FIG. 3 depicts a case in which thereference voltage V2 is set to zero in the current detection circuit 230in FIG. 2.

[0092] The flywheel current detection signal IF is inverted by theinverter 216 of the overcurrent protection circuit 220. Since the risingof the inverter 216 output is detected by the edge detection circuit217, an edge detection pulse IFE is outputted from the edge detectioncircuit 217 at the timing at which the flywheel current I-L decreases tozero, as depicted in FIG. 3H.

[0093] This edge detection pulse IFE is applied to the reset terminal Rof the flip-flop 218, and the overcurrent detection signal OCD outputtedfrom the overcurrent detection circuit 221 is applied to the setterminal S of the flip-flop 218. Thus, a low-level gate signal GS2 isoutputted from the inverted output terminal of the flip-flop 218 fromthe time when overcurrent is detected by the overcurrent detectioncircuit 221 until the time the flywheel current I-L decreases to zero,as depicted in FIG. 31.

[0094] The overcurrent detection signal OCD outputted from theovercurrent detection circuit 221 is applied to the set terminal S ofthe flip-flop 213, and the clock pulse CK is applied to the resetterminal R thereof Thus, a low-level gate signal GS1 is outputted fromthe inverted output terminal of the flip-flop 213 from the time whenovercurrent is detected by the overcurrent detection circuit 221 untilthe time the next clock pulse CK rises, as depicted in FIG. 3D.

[0095] This gate signal GS1 is applied to the AND circuit 214 in whichthe drive pulse DP outputted from the PWM circuit 210 is applied toanother input terminal (see FIG. 3F), and the above-mentioned gatesignal GS2 is applied to the AND circuit 215 in which the output of theAND circuit 214 is applied to another input terminal. Therefore, thedrive pulse DP outputted from the PWM circuit 210 is masked from thetime when overcurrent is detected by the overcurrent detection circuit221 until the time the flywheel current I-L decreases to zero.

[0096] With the above operation, the drive pulse DP′ depicted in FIG. 3Jis outputted from the inverter 219 for inverting the output of the ANDcircuit 215, and the transistor 20 is controlled by the drive pulse DP′even if the on-duty time of the drive pulse DP′ decreases to the minimumtime Tmin as depicted in FIG. 3J. Therefore, an overcurrent does notgenerated regardless of the saturation current value of the inductor 30until a breakdown occurs in the circuit elements by currentsuperposition in the inductor 30.

[0097] Further, in the switching power supply circuit of the presentembodiment, the transistor 20 is turned on after the flywheel currentI-L goes to zero or decreases to a value where an overcurrent is notgenerated until a breakdown occurs in the circuit elements by currentsuperposition. Therefore, startup time can be sufficiently shortenedduring startup, and responsiveness to sudden load fluctuations is notdeteriorated.

[0098]FIG. 4 is a circuit diagram depicting another embodiment of theswitching power supply circuit according to the present invention.

[0099] The depicted in FIG. 4 is a synchronous rectification-typeswitching power supply circuit in which the diode 40 of the switchingpower supply circuit depicted in FIG. 1 is substituted with a transistor(field-effect transistor) 91 whose on/off state is controlled by theoutput of the PWM circuit 210. In the switching power supply circuit ofthis embodiment, the overcurrent detection circuit of the controlcircuit 200 detects the flywheel current I-L by using the resistance ofthe transistor (field-effect transistor) 91 in on state. Theconfiguration other than the above is the same as that of the switchingpower supply circuit depicted in FIG. 1.

[0100] Specifically, the transistor 91 is turned on when the transistor20 is turned off by the output of the PWM circuit 210. The flywheelcurrent I-L then flows via the transistor 91 that has been turned on,and the overcurrent detection circuit of the control circuit 200 detectsthis flywheel current I-L using the resistance of the transistor 91 inon state.

[0101]FIG. 5 is a circuit diagram depicting the detailed structure ofthe overcurrent protection circuit 220 and current detection circuit 230of the control circuit 200 depicted in FIG. 4.

[0102] The configurations of the overcurrent protection circuit 220 andthe current detection circuit 230 are the same as those of theovercurrent protection circuit 220 and current detection circuit 230depicted in FIG. 2 except that the source of the transistor 91 isconnected to the positive terminal of the operational amplifier 231 inthe current detection circuit depicted in FIG. 5.

[0103] Specifically, in the configuration of this embodiment, anelectric potential that corresponds to a voltage drop across theresistance of the transistor 91 in on state caused by the flywheelcurrent I-L is inputted to the positive terminal of the operationalamplifier 231, and an electric potential (GND−V2) in which a referencevoltage V2 is subtracted from the ground potential is inputted to thenegative terminal thereof, a high-level flywheel current detectionsignal IF is outputted from the operational amplifier 231 when thevoltage drop across the on-resistance of the transistor 91 caused by theflywheel current I-L exceeds the electric potential (GND−V2).

[0104]FIG. 6 is a circuit diagram depicting a modification of thecurrent detection circuit 230 depicted in FIG. 5.

[0105] The current detection circuit depicted in FIG. 5 is configuredsuch that the source of the transistor 91 inputs directly to thepositive terminal of the operational amplifier 231 so as to detect theflywheel current I-L. On the other hand, in the current detectioncircuit 230 depicted in FIG. 6, a transistor (field-effect transistor)233 is provided that constitutes a current mirror circuit with thetransistor 91, a resistor 234 is connected to the drain of thetransistor 233, and the flywheel current I-L is detected by using thevoltage drop across the resistor 234. Other aspects of the configurationare the same as that depicted in FIG. 5.

[0106] Specifically, in the configuration of FIG. 6, the output of thePWM circuit 210 is applied to both the transistor 233 and the transistor91 together, and a current corresponding to the flywheel current I-Lflowing through the transistor 91 flows in the transistor 233.

[0107] The current flowing through the transistor 233 is then detectedby the voltage drop across the resistor 234 and applied to the positiveterminal of the operational amplifier 231. Also, because electricpotential (GND−V2) in which a reference voltage V2 is subtracted fromthe ground potential is inputted to the negative terminal of theoperational amplifier 231, a high-level flywheel current detectionsignal IF is outputted from the operational amplifier 231 when thevoltage drop across the resistor 234 caused by the flywheel current I-Lexceeds the electric potential (GND−V2).

[0108] The same overcurrent protection as in the configurations depictedin FIGS. 1 and 2 can be performed by means of the above-mentionedconfigurations depicted in FIGS. 4 and 5.

[0109] Also, a positive potential may be used as the reference voltageV2 (second reference value) by connecting the power supply 232 to thenegative terminal of the operational amplifier 231 for the currentdetection circuit 230, as depicted in FIG. 7. In this case, theconfiguration can be simplified because the need is eliminated forpreparing a separate power supply 232 such as is shown in the currentdetection circuit 230 of FIG. 2.

[0110] Further, as depicted in FIG. 8, the negative terminal of theoperational amplifier 231 may be set to the ground potential, and a zeropoint may be used as the reference voltage V2. The simplestconfiguration can be achieved in this case because there is no need forboth a negative power supply and a positive power supply.

[0111] When a positive potential or negative potential is used as thereference voltage V2, the threshold value needed to eliminate currentsuperposition may be set as the electric potential according to thesuperposition characteristics of the inductor being used. For example, avalue may be set that is distant from the zero potential when aninductor is used in which current superposition does not easily occur,and a value may be set that is in the vicinity of the zero potentialnear a complete discharge when using an inductor in which currentsuperposition occurs easily.

What is claimed is:
 1. A switching power supply circuit having aswitching element for controlling on/off state of an input power supplyaccording to a drive pulse generated on the basis of a clock pulsehaving a prescribed period, wherein an electric current is allowed toflow from the input power supply to a load by controlling the switchingelement to be in on state, and a flywheel current is allowed to flow tothe load by controlling the switching element to be in off state; theswitching power supply circuit comprising: overcurrent detecting meansfor detecting that the current flowing through the switching element hasreached a first reference value; flywheel current detecting means fordetecting that the flywheel current has reached a second referencevalue; and an overcurrent protection circuit for performing anovercurrent protection operation of turning the switching element offbased on a detected output of the overcurrent detecting means, and forturning the switching element on during the overcurrent protectionoperation at a timing of the clock pulse after the flywheel currentdetected by the flywheel current detecting means has reached the secondreference value.
 2. The switching power supply circuit according toclaim 1, wherein the flywheel current detecting means detects theflywheel current based on an output of a current detection resistorprovided in a flywheel current path.
 3. The switching power supplycircuit according to claim 1, wherein the flywheel current detectingmeans comprises a second switching element provided in the flywheelcurrent path and being turned on and off in synchronism with thefirst-mentioned switching element, and detects the flywheel currentusing a resistance of the second switching element in on state.
 4. Theswitching power supply circuit according to claim 1, wherein theflywheel current detecting means further comprises a third switchingelement provided in the flywheel current path and constitutes a currentmirror together with the second switching element that is turned on andoff in synchronism with the first-mentioned switching element, wherebythe flywheel current detecting mean detects the flywheel current basedon the current flowing through the third switching element.
 5. Theswitching power supply circuit according to claim 1, wherein the secondreference value is set to a value of zero or above.
 6. A switching powersupply circuit comprising: a switching element provided with a sourceterminal, a drain terminal and a gate terminal; a power supply sourceconnected to the source terminal; a load connected to the drainterminal; a control circuit connected to the gate terminal; an inductordisposed in a current path from the drain terminal to the load; a firstcurrent detection resistor disposed in a current path from the sourceterminal to the load; a first comparator connected to the first currentdetection resistor; a second current detection resistor disposed in acurrent path from the drain terminal to the ground terminal; and asecond comparator connected to the second current detection resistor. 7.The switching power supply circuit according to claim 6, wherein thesecond comparator uses a ground potential or positive potential as areference.
 8. An overcurrent protection method for a switching powersupply circuit having a switching element for controlling on/off stateof an input power supply according to a drive pulse generated on thebasis of a clock pulse having a prescribed period, wherein an electriccurrent is allowed to flow from the input power supply to a load bycontrolling the switching element to be in on state, and a flywheelcurrent is allowed to flow to the load by controlling the switchingelement to be in off state; the overcurrent protection methodcomprising: detecting that the current flowing through the switchingelement has reached a first reference value; detecting with anovercurrent detecting means that the current flowing through theswitching element has reached a first reference value; detecting withflywheel current detecting means that the flywheel current has reached asecond reference value; performing an overcurrent protection operationfor turning off the switching element based on a detected output of theovercurrent detecting means; and turning on the switching element duringthe overcurrent protection operation at a timing of the clock pulseafter the flywheel current detected by the flywheel current detectingmeans has reached the second reference value.